because critical limits have been violated the reason why the process
went out of control must be established and measures to
prevent recurrence should be implemented and documented.
4. Statistics of sampling
For sampling plans in which detection of positives is not
accepted (c ¼ 0), which is often the case for pathogens, the equation
to determine the probability of detection is:
Pdetect
n; c ¼ 0; Pdefective
¼ 1
1 Pdefective
n
(1)
In words this equation can be explained as follows: The probability
that one sample is not defective is 1 minus the probability of a
defective. For all n samples to be not defective, the probability is
this term to the power n. So (1-Pdefective)n is the probability that all n
samples are not defective. One minus this value is the probability
that one or more of the samples are contaminated, so that the organism
is detected in one or more samples.
This equation shows that the performance of sampling is often
rather poor, definitely with a low rate of defective (i.e., contaminated)
products (Table 2) and even when large numbers of sample